Multiam star polyesters were synthesized by growing poly(e- caprolactone) (PCL) arms from hyperbranched polyesters cores of different molecular weight and used as polymeric modifiers in UV-curable cationic formulations based on a biscycloaliphatic epoxy resin. The effect of the multiarm stars on the curing kinetics has been investigated by real-time FTIR. The thermal-mechanical properties of the photocured thermosets have been studied with calorimetry and dynamomechanical and thermogravimetric analysis. Impact strength tests have been performed to assess their effect on the toughness of the cured materials. An accelerative effect of these modifiers has been observed as a consequence of the participation of the hydroxyl groups of the modifiers in the cationic curing of the epoxy resin. A modest increase in toughness accompanied by a decrease in the glass transition are observed, as a consequence of the incorporation of the modifiers into the network structure, leading to homogeneous, in situ reinforced materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 40005. Copyright © 2014 Wiley Periodicals, Inc.